1. Field of the Invention
The present invention relates to a beam-spot position compensation method for image forming device, such as digital copier, laser printer or laser facsimile, and relates to an optical scanning device and a multi-color image forming device using the beam-spot position compensation method.
2. Description of the Related Art
Generally, the optical scanning device is widely known in connection with the laser printer or the like. The optical scanning device is provided in which the light beam from the light source is deflected by the optical deflector, it is focused on the scanned surface by the scanning/focusing optical system including the fθ lens, so that the optical spot is formed on the scanned surface, and the optical scanning of the scanned surface is carried out using this optical spot. The scanned surface is, for example, the photoconductive surface of the photoconductor or the like.
The optical deflector is, for example, the commonly used polygon scanner in which the deflection surface is rotated at equal angular velocity. The light source is, for example, the semiconductor laser which is modulated by a fixed frequency.
When the optical scanning device is constituted using such light source and such optical deflector and the optical scanning of the scanned surface, such as the photoconductor, is carried out, the beam-spot position is not arranged at equal intervals, and the scanning speed is not constant.
For this reason, in order to arrange the beam-spot position at equal intervals and set the scanning speed at the fixed level when performing the optical scanning, the uniform optical scanning of the scanned surface is attained by performing the compensation using the scanning/focusing optical system, such as the fθ lens.
However, there is the limitation in the compensation of the scanning speed using the fθ lens. The scanning speed cannot be completely set at the fixed level, and the scanning speed variance may arise. Thus, the sparseness or denseness of the beam-spot position spacing takes place, and it is desirable that the beam-spot position spacing be set to the fixed interval.
Furthermore, the sparseness or denseness of the beam-spot position spacing is increased according to the manufacture error of the fθ lens.
If the sparseness or denseness of the beam-spot position spacing occurs, the distortion of the image arises, which causes the degradation of image quality.
Moreover, the color image forming device uses two or more fθ lenses, and, due to the manufacture error of the fθ lens and others, the sparseness or denseness of the beam-spot position spacing which is varied for each of the colors arises, and consequently the color deviation arises.
There is the known technique which compensates the sparseness or denseness of the beam-spot position spacing. According to the known technique, the frequency of the pixel clock is varied and the beam-spot position along the scanning line is compensated. Such technique is known from Japanese Laid-Open Patent Applications No. 11-167081 and No. 2001-228415, and Japanese Patent No. 3512397 (which corresponds to Japanese Laid-Open Patent Application No. 2003-098465).
However, the conventional technique is directed to the frequency modulation method in which the frequency of the pixel clock is varied. Generally, the pixel clock control unit for the frequency modulation method is complicated in the structure, and there is the problem that when the width of the frequency modulation becomes very small, it is difficult to perform the pixel clock control suitably, and the compensation of the beam-spot position with high accuracy cannot be performed. Moreover, there is the problem that the algorithm for obtaining the compensation data needed for performing the compensation is very complicated.